This invention relates to an automatic sewing machine having a feeding device for imparting a continuous movement to a workpiece during the sewing process and a sewing head including a needle bar arranged to be driven in oscillatory manner by an arm shaft by means of a gear drive.
Such sewing machines, known e.g. from U.S. Pat. No. 4,157,686, have feeding devices comprising a linkage system formed by a four-bar linkage, whose two guide levers associated with a fixed fulcrum in each case engage by means of a guide roller or the like in in each case one control groove on the top or bottom of the control disc. Thus, on rotating the control disc, the workpiece holder connected to the off-drive lever is guided in a plane along a two-dimensional groove predetermined by the course of the two control grooves. Such feeding devices have proved to be extremely satisfactory in practice, because they are constructionally simple and can be easily supervised and because for producing a different seam configuration, i.e. for producing a different movement of the workpiece holder, it is merely necessary to use a control disc with different control grooves. However, a problem of such sewing machines is that the workpiece is also moved during the insertion of the needle, so that a so-called needle deflection is produced, which can lead to collisions between the needle and the hook tip and even to the needle breaking.
In the case of the automatic sewing machine known from German Patent Specification No. 2733397, for reducing this problem, an additional gear with a further driving arrangement is positioned upstream of the feeding device and as a result the movement of the sewing material holder during the insertion of the needle is reduced. However, such a design is constructionally complicated. As a result of the oscillatory drive of the feeding device, humming occurs at least occasionally and also the gear must be constructed free from play.
U.S. Pat. No. 480298 discloses an eccentric gear drive for the needle bar of a sewing machine, by means of which the needle is accelerated in the lower area. The object of this measure is to gain time in order to be able to correctly operate the recpetion mechanism and the sewing material transport.